1. Field of the Invention
This invention relates to an image forming apparatus including a fixing device for fixing, by heating, a toner image on a recording material.
2. Description of the Related Art
Heretofore, temperature detection means in a fixing device has been situated at a central portion in the longitudinal direction of the surface of a fixing roller heated by a heating source. In this case, toner may adhere on the surface of the temperature detection means or a flaw may be produced in the roller when the temperature detection means contacts the roller, and black stripes may therefore be produced in an image. In order to solve these problems, there has been known a device in which temperature detection means is disposed at an end portion of a fixing roller (Japanese Utility Model Public Disclosure (Chalk) No. 59-147168 (1984)).
However, even if temperature detection means is disposed at an end portion of a heating (fixing) roller to detect the temperature at the end portion, exact temperature control of a fixing roller cannot be performed without taking into consideration the temperature of a central portion of the roller where a recording material passes, causing failure in fixing.
More specifically, images are typically formed using three kinds of recording materials A, B and C (A&gt;B&gt;C in width) having different widths (the length in the axial direction of the fixing roller). As shown in FIG. 5, if temperature control means is controlled so as to maintain a temperature which is most suitable at a position where a temperature detector is disposed, the surface temperature at a central portion of the fixing roller has variations. It is therefore impossible to maintain a temperature which is most suitable for fixing. This is because the temperature at the central portion decreases to cause failure in fixing in the case of the recording material C having the smallest size, and the temperature at the central portion becomes too high so as to produce high-temperature offset in the case of the recording material A having the largest size when a temperature has previously been set in accordance with the recording material C.